This invention relates to a magnetic recording medium comprising a thin ferromagnetic metal film as a magnetic recording layer, and more particularly to a magnetic recording medium having a protective layer high in durability and weatherability, and having a good running property.
Magnetic recording media commonly used in the art are of "coated type", and are typically prepared by applying to a nonmagnetic support a dispersion of a particulate magnetic material such as a magnetic powder or ferromagnetic alloy powder, e.g., .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a berthollide (e.g., an alloy of variable proportions) or .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, and CrO.sub.2 in an organic binder such as vinyl chloride-vinyl acetate copolymer, styrene-butadiene copolymer, epoxy resin, or polyurethane resin. As the demand for recording in higher density has increased, reserch has increased including research directed to magnetic recording media of "non-binder type", wherein a magnetic recording layer comprising a thin ferromagnetic metal film formed, e.g., by a vapor deposition technique such as vacuum evaporation, sputtering or ion plating, or a plating technique such as electroplating or electroless plating, rather than by using a binder, and efforts have been made to develop a commercial product of this type.
The conventional magnetic recording medium of coated type typically uses a magnetic material made of a metal oxide having a smaller saturation magnetization than a ferromagnetic metal, and using such a material it becomes futile to try to increase the recording density beyond a certain point by reducing the thickness of the magnetic layer, because it results in low signal output. Furthermore, the manufacture of such a recording medium involves complicated procedures and requires additional facilities for solvent recovery or pollution control. The magnetic layer of the magnetic recording medium of the non-binder type is made by forming a thin layer of a ferromagnetic metal having a greater saturation magnetization than the metal oxide, without using a nonmagnetic substance such as binder. Accordingly, a very thin magnetic recording medium suitable for high density recording can be produced by such a procedure.
It is considered desirable, both theoretically and empirically, that a magnetic recording medium capable of high-density recording should have high coercive force and should have a thickness as small as practically possible, and it is thought that the magnetic recording medium of the non-binder type can be made one-tenth as thick as the magnetic recording medium of coated type, and also have a higher saturated magnetic flux density than the coated type.
Two important requirements that must be met by a magnetic recording medium, and particularly a magnetic recording type, using a thin ferromagnetic metal film as the magnetic layer are that it should have great resistance to corrosion and wear, and exhibit stability during running of the tape. During recording, reproduction, and erasure of magnetic signals, the magnetic recording medium moves very quickly over the magnetic head, and in such mode, it must run smoothly and produce stable output without being worn or damaged by contact with the head. It is also required that the signal recorded not be attenuated or lost because of corrosion or other phenomena taking place during storage of the magnetic recording medium.
Few ferromagnetic metal layers are capable of withstanding the severe conditions under which magnetic recording and reproduction are performed, and therefore protective layers are desirably formed on the surface of such ferromagnetic metal layers. Effective protective layers must be made of a material which is hard, forms a thin, uniform layer on the substrate, and is strongly bonded to the substrate. A coating which easily separates from or wears off on the substrate under hostile environments provides insufficient protection for the underlying magnetic recording layer, and also causes problems due to fragments of the separating coating. Another proposed solution to the problem involves forming a protective layer of electroplated rhodium. Other methods for providing a protective layer on the magmetic layer of the magnetic recording medium of non-binder type include: (1) a technique of oxidizing the surface of a thin cobalt-containing ferromagnetic metal film by exposing it to a suitable temperature and humidity, as described in Japanese Patent Publication No. 20025/67 and U.S. Pat. No. 3,353,166; (2) a technique of contacting a thin magnetic alloy film with nitric acid, treating the same with heat to form an oxide film on the surface, and impregnating the film with a lubricant, as described in British Pat. No. 1,265,175; and (3) a technique for depositing, by vacuum evaporation under a suitable degree of vacuum, a vapor of chromium on the surface of a thin ferromagnetic metal film to thereby form a layer composed of a mixture of chromium and chromium oxide, as described in Japanese Patent Publication No. 4393/70. Protective layers comprising nonmagnetic metals such as Si, Al, Ti, Sn, In, or Zn or oxides or nitrides thereof are also known. All these protective layers have achieved some, but nevertheless unsatisfactory, improvement in wear resistance and durability, and the running property of magnetic recording media coated with such protective layers is far from being satisfactory.